The invention relates to a bone-anchoring assembly. More particularly, the invention relates to self-cutting, bone anchoring elements that are attachable to osteosynthesis fixation plates or longitudinal support bars for the fixation of bone segments, such as vertebra.
Fixed implants such as bone plates, longitudinal support bars, pedicle screws, and bone anchoring assemblies increasingly are used in osteosynthesis applications. Such devices are useful for treating fractures of bones, for anchoring bone segments, or for providing support to bones weakened from disease or defect.
One such implant for the relative affixation of bone segments or vertebrae is disclosed in the German utility model DE 297 10 979 to Aesculap. The implant comprises a bone anchoring element that is insertable into a bone segment. The bone anchoring element can then be mounted using a detachable ball clamp to a connection element. The connection element can, in turn, be clamped to a longitudinal support or to another bone anchoring element. By connecting several bone anchoring elements together, bone segments or vertebra can be rigidly connected together. As disclosed in the German utility model, the anchoring elements are in the form of hollow, cylindrical bone screws that are externally threaded and fitted with radial boreholes located between the threads. The drawback, however, to this form of bone anchoring is that a seating duct must be drilled in the bone before the hollow, cylindrical anchoring elements can be inserted.
A similar shortcoming is present in the spinal interbody fusion assembly disclosed in U.S. Pat. No. 6,156,037 to LeHuec et. al. The spinal interbody fusion apparatus disclosed in this patent comprises an interbody fusion cage that has an external thread present along the entire length of the cage and a threaded stem located at the domed posterior end of the cage. A contoured plate threadably receives the cage stem and connects this cage to other interbody fusion cages. However, as mentioned earlier, the shortcoming to this apparatus is that a bone duct or bore must be drilled prior to the insertion of the interbody fusion cage.
A bone segment affixation implant comprising a hollow-cylindrical bone screw is disclosed in U.S. Pat. No. 5,015,247 to Michelson. This bone screw is designed for insertion in the intervertebral space and also consists of a hollow cylinder with an external thread and radial passages between the threads. Again, the drawback to this form of bone anchoring is that this implant must be inserted into a bone duct or bone borehole that has been previously drilled.
U.S. Pat. No. 4,537,185 to Stednitz also discloses a bone fixation screw with a hollow central cavity, a circular cylindrical anchoring section, a connecting element at the posterior end of the screw, and an external thread. In addition, this screw also has cutting teeth at the anterior end of the screw and the threads that are located on the external surface of the screw are self-cutting. The cutting teeth on the anterior portion of the screw and the external cutting threads allow the screw to be inserted into a bone segment without having to drill a bone duct or bone borehole. However, the drawback to using this bone anchoring is when the bone has been subject to osteoporosis or similar degenerative disease. Typically, the cortical region of the bone remains but the spongy portion of the bone is receded thus anchoring the screw into the spongy portion is not possible. Here, the circular cylindrical anchoring section of the bone screw is partially located in the spongy portion of the bone thus anchoring the bone screw in the bone is difficult. Also, the external thread of the bone screw extends into the anterior end of the screw which typically is located in the spongy portion of the bone. Having the thread extend into the spongy portion of the bone is disadvantageous because the micro-motion shear and notch effects that result from a self-cutting thread can harm the spongy region.
Another bone fixation screw having a hollow central cavity, a circular cylindrical anchoring section, a connecting element at the posterior end of the screw, anterior cutting teeth, and a self-cutting external thread is disclosed in U.S. Pat. No. 5,129,901 to Decoste. However, this bone fixation screw suffers from the same shortcoming as discussed above with the Stednitz bone screw.
In light of the foregoing, it is clear that there exists a need for an improved bone anchoring element.
The present invention relates to a bone anchoring assembly having at least one bone anchoring element capable of being attached to at least one osteosynthesis plate or bar for the fixation of bone segments. The at least one bone anchoring element preferably includes a circular-cylindrical hollow body fitted with cutting teeth at the anterior or lower end of the element, a connecting element at the posterior or upper end of the body for coupling to another fixation element, a plurality of radial borehole passages located on the body of the bone anchoring element, a flange located at the upper end of the body for limiting insertion depth of the bone anchoring element, and a self-tapping, external thread that extends over a portion of the bone anchoring element. Preferably, the connecting element is either circular-cylindrical or hexagonally-cylindrical in shape and houses a borehole that is configured and dimensioned to receive a fastener.
In one preferred embodiment, the bone anchoring element consists of a circular-cylindrical anchoring body fitted at one end with tangentially arranged cutting teeth. The number of cutting teeth range between 10 and 40, but preferably are between 25 and 35, with a clearance angle of any where from 5xc2x0 to 40xc2x0, but preferably between 18xc2x0 to 28xc2x0. The cutting edge of the teeth, typically, are at angle between 30xc2x0 to 60xc2x0 from the longitudinal axis of the bone anchoring body but preferably are at an angle between 40xc2x0 and 50xc2x0, with the cutting corner of the teeth located at the outside surface of the anchoring body. Preferably, the rake of the cutting teeth is between 25xc2x0 and 35xc2x0. At the other end of the anchoring body is a connecting element for coupling the anchoring body to another implantable element such as a fixation plate, an adjustment plate, or longitudinal supports. The connecting element is designed to couple with another implantable element in a pivoting fashion which allows the anchoring body to pivot with respect to the implantable element.
In another preferred embodiment, the outside surface of the anterior or lower portion of the anchoring element does not contain an external thread allowing for a smooth surface with radial borehole passages. The radial borehole passages allow the osteoinductive material located within the hollow anchoring body to fuse with the bone located outside the anchoring body. In addition, the radial borehole passages reduce the amount of material needed to create the implant, thereby substantially lowering the total weight of the bone anchoring element.
In a further preferred embodiment, the bone anchoring apparatus comprises at least two bone anchoring elements and at least one plate fitted with means to receive the connecting elements of the bone anchoring elements. The connecting element receiving means essentially consists of boreholes located in the plate appropriately sized to allow the connecting elements to be pivotably supported in the plate and detachably affixed to the plate by bone anchoring fasteners such as screws or nuts. Preferably, the connecting elements are pivotably connected to the plate so as to allow the bone anchoring elements to pivot between 60xc2x0 and 120xc2x0. The connecting element receiving means are located throughout the plate in such a manner as to allow the at least two bone anchoring elements coupled to the plate to be displaced between 10 mm to 80 mm from each other along a single axis, but preferably be displaced between 20 mm to 60 mm from each other.
In another preferred embodiment, the bone anchoring apparatus further comprises at least two plates wherein each plate has an elongated central channel that extends along a central axis, across most of the length of the plate, capable of receiving a fastener that will affix the plates together at any distance along the central axis within the central channel. The fastener is preferably a screw or a bolt and the plates preferably have textured surfaces at their respective points of contact to prevent slippage of the plates with respect to each other and to increase the stability of the affixed plates. In addition, the plates preferably have lateral lugs to further prevent slippage and to prevent rotation of the plates with respect to each other.
In a further preferred embodiment, the bone anchoring apparatus further comprises two annular disks and at least one plate, wherein one disk is fixably mounted to the plate and the second disk is mounted within the elongated channel of the plate to allow variable spacing between the two disks. Each disk receives the connecting element of the bone anchoring element to pivotably couple the bone anchoring element to the plate through the disk.